1. Field of the Invention
The present invention relates to a method for servo control of an optical disk reproducing/recording device, and more particularly to a method for control of a feed motor for an optical disk reproducing/recording device.
2. Description of the Related Art
Shown in FIG. 1 is a general optical disk reproducing device, comprising an optical pickup 2 for reproducing recorded signals from an optical disk 1, an R/F unit 3 for outputting servo error signals and binary signals through combining signals reproduced from the optical disk 1 by the optical pickup 2, a synchronization unit 7 for creating a clock signal synchronized with the binary data outputted from the R/F unit 3, a digital signal processing unit 4 for processing the binary data stream received from the R/F unit 3 to retrieve digital data using the synchronization clock, a sled motor 11 for moving the optical pickup 2 in the radial direction, a spindle motor 12 for rotating the optical disk 1, a driver 8 for driving the sled motor 11 and the spindle motor 12, a servo unit 6 for providing control input to the driver 8, and a microcomputer 9 for supervising overall operation of the servo unit 6 and the digital signal processing unit 4.
Once the optical disk 1 is inserted into a disk tray, which is a part of the optical disk reproducing device mechanism, the optical disk 1 is clamped by a clamping device. Then, controlled by the servo unit 6, the drive 8 provides the spindle motor 12 with electric current, thereby rotating the optical disk 1. Provided with DC voltage by the drive 8, the sled motor 11 moves the optical pickup 2 a predetermined initial position.
To reproduce signals recorded on the optical disk 1, the optical pickup 2 with a laser diode and photo diodes installed in it, forms a laser beam spot on the recording layer of the optical disk 1 by irradiating a laser beam from the laser diode. The laser beam reflected from the optical disk 1 is collected by the photo diodes and converted into electric signals. A tracking error signal is generated from a combination of the electric signals in the R/F unit 3. Using the tracking error signal, the servo unit 6 creates tracking control input to make the laser beam spot follow a target track. The control input is applied to the driver 8 that provides electric current for the tracking actuator which moves an objective lens.
As the tracking actuator moves the objective lens in the radial direction, the center of the objective lens begins to deviate from the optical axis, which creates offset voltage in the tracking error. Therefore, a low-pass filtered tracking error signal indicates the distance between the center of the objective lens and the optical axis. The sled motor 11 is controlled to move the optical pickup 2 in such a way that the magnitude of the center error decreases if the deviated distance has just approached to the maximum operating range of the tracking actuator, thereby moving the optical pickup 2 toward the optical axis.
The servo unit 6 creates control input for the DC sled motor 11 based on the nominal model of the sled motor 11. The characteristics of the sled motor 11, however, may change as the operating environment such as temperature varies. If so, the required control accuracy for the sled motor 11 may not be attainable, although the servo unit 6 works as designed. In this case, the objective lens is not positioned around the optical axis as shown in FIG. 2, which implies that the operating range of the tracking actuator cannot be fully utilized or may result in tracking failure in the worst case.
It is an object of the present invention to provide a method and apparatus for servo control of an optical disk recording/reproducing device, capable of maintaining the center of the object lens around the optical axis within a prescribed range.
A method for servo control of an optical disk reproducing/recording device in accordance with the present invention comprise the steps of detecting the distance between the center of an objective lens and an optical axis, examining whether the detected distance exceeds a prescribed range, and changing the magnitude of each drive signal by a microstep depending upon the examination result and applying the drive signals to a sled motor for moving an optical pickup containing the objective lens.
An apparatus for servo control of an optical disk reproducing/recording device in accordance with the present invention comprises detection means for detecting a signal indicative of the distance between the center of an objective lens and an optical axis, determination means for determining the direction to which the center of the objective lens deviates from the optical axis and the magnitude of the deviation, control means for changing the magnitude of each drive signal by a microstep depending upon the determination result and outputting the drive signals, and drive means for driving a sled motor for moving an optical pickup containing the objective lens in response to the drive signals.
In the present invention, the distance between the center of the objective lens and the optical axis is estimated while reproduction of recorded data is performed. If the estimated distance exceeds a predetermined range, drive pulses are applied to the stepping motor, thereby moving the optical pickup in such a way that the distance decreases.